The microcirculation of the renal medulla traps NaCI and urea deposited to the interstitium by the loops of Henle and collecting ducts and distributes blood flow to a hypoxic region of the kidney. Substantial evidence has linked medullary perfusion to renal regulation of salt and water excretion and genesis of acute renal failure. Descending vasa recta (DVR) are 15 um diameter arteriolar microvessels through which blood flow reaches the medulla. DVR are surrounded by smooth muscle / pericytes that impart contractile properties to regulate vasoactivity. We have now established methods to study Ca 2+ signaling and channel architecture of the DVR pericyte. Using those methods we have shown that pericytes depolarize through activation of a Ca 2+-dependent CI- conductance and inhibition of K+ channels. These events accompany angiotensin II induced vasoconstriction. To advance our understanding of the physiology of the DVR pericyte, we propose the following aims: Aim 1) We will examine the physiological properties of the Ca 2+-dependent CI- conductance that regulates pericyte membrane potential. We will determine the anion selectivity and sensitivity to inhibitors of whole cell currents and the 11 pS CICa channel we previously identified. We will test the roles of protein kinase C and protein kinase A in the activation and inactivation of the rat pericyte CICa channel. Aim 2) We will examine pericyte Kv channel suppression by angiotensin II (Angll). We will test the cation selectivity of Angll suppressible whole cell currents. We will determine the cation selectivity and activation mechanisms of a 58 pS Kv channel that Angll suppresses in cell attached patches. Aim 3) We will test the role of 20-HETE to mediate Angll induced DVR vasoconstriction and Kv channel inhibition. We will test the role of 20-HETE to regulate pericyte membrane potential and suppress K+ channels. Aim 4) We will determine which pathways participate in Ca 2+ entry in DVR pericytes. We will examine the current voltage characteristics and cation selectivity of Ca 2+ entry pathways opened by Ca 2+store depletion. We will test the hypothesis that voltage operated Ca 2+ entry participates in pericyte signaling. We will determine the role of VOCC and nonselective cation channels in Angll signaling and test a role for protein kinase C in their activation.